Electrolyte additive and electrolyte for lithium secondary battery including the same

ABSTRACT

An electrolyte additive composition of the present invention may improve high-rate charge and discharge characteristics and high-temperature storage and life characteristics of a lithium secondary battery when the electrolyte additive composition is used in an electrolyte while including a novel borate-based lithium compound as well as a nitrile-based compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national phase entry under 35 U.S.C. § 371of International Application No. PCT/KR2018/003133 filed Mar. 16, 2018,which claims priority from Korean Patent Application No. 10-2017-0034037filed Mar. 17, 2017, all of which are incorporated herein by reference.

TECHNICAL FIELD Technical Field

The present invention relates to an electrolyte additive and anon-aqueous electrolyte for a lithium secondary battery including thesame.

Background Art

As the miniaturization and weight reduction of electronic devices arerealized and the use of portable electronic devices is common, researchinto secondary batteries having high energy density, as power sources ofthese devices, has been actively conducted.

The secondary battery includes a nickel-cadmium battery, a nickel-metalhydride battery, a nickel-hydrogen battery, and a lithium secondarybattery, and, among these batteries, research into lithium secondarybatteries, which not only exhibit a discharge voltage two times or morehigher than a typical battery using an aqueous alkaline solution, butalso have high energy density per unit weight and are rapidlychargeable, has been emerged.

A lithium metal oxide is used as a positive electrode active material ofa lithium secondary battery, and lithium metal, a lithium alloy,crystalline or amorphous carbon, or a carbon composite is used as anegative electrode active material. A current collector is coated withthe active material of appropriate thickness and length or the activematerial itself is coated in the form of a film, and the resultantproduct is then wound or stacked with an insulating separator to prepareelectrodes. Thereafter, the electrodes are put into a can or a containersimilar thereto, and a secondary battery is then prepared by injectingan electrolyte.

Charge and discharge of the lithium secondary battery is performed whilea process of intercalating and deintercalating lithium ions from alithium metal oxide positive electrode into and out of a graphitenegative electrode is repeated. In this case, since lithium is highlyreactive, the lithium reacts with the carbon electrode to form Li₂CO₃,LiO, or LiOH, and thus, a film may be formed on the surface of thenegative electrode. The film is referred to as “solid electrolyteinterface (SEI)”, wherein the SEI formed at an initial stage of chargingmay prevent a reaction of the lithium ions with the carbon negativeelectrode or other materials during charge and discharge. Also, the SEIonly passes the lithium ions by acting as an ion tunnel. The ion tunnelmay prevent the collapse of a structure of the carbon negative electrodedue to the co-intercalation of the carbon negative electrode and organicsolvents of the electrolyte having a high molecular weight whichsolvates the lithium ions and moves therewith.

Thus, in order to improve high-temperature cycle characteristics andlow-temperature output of the lithium secondary battery, a robust SEImust be formed on the negative electrode of the lithium secondarybattery. Once the SEI is formed during initial charge, the SEI mayprevent the reaction of the lithium ions with the negative electrode orother materials during repeated charge and discharge caused by thesubsequent use of the battery and may act as an ion tunnel that onlypasses the lithium ions between the electrolyte and the negativeelectrode.

Conventionally, with respect to an electrolyte which does not include anelectrolyte additive or includes an electrolyte additive having poorcharacteristics, it was difficult to expect the improvement oflow-temperature output characteristics due to the formation of anon-uniform SEI. Furthermore, even in a case in which the electrolyteadditive is included, since the surface of the positive electrode isdecomposed or the electrolyte causes an oxidation reaction during ahigh-temperature reaction due to the electrolyte additive when an amountof the electrolyte additive added may not be adjusted to the requiredamount, irreversible capacity of the secondary battery may ultimately beincreased and output characteristics may be reduced.

Thus, there is a need to develop a compound which may be used as anelectrolyte additive for improving overall performance, such ashigh-rate charge and discharge characteristics, high-temperatureperformance characteristics, and life characteristics, of the battery byforming a robust SEI on the negative electrode.

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a novel electrolyte additivecapable of improving high-rate charge and discharge characteristics,suppressing an amount of gas generated during high-temperature storage,and improving life characteristics, and an electrolyte additivecomposition which may improve the above-described performance of alithium secondary battery by including the novel electrolyte additive aswell as an additive capable of having a synergistic effect onperformance improvement when used together.

Technical Solution

According to an aspect of the present invention, there is provided anelectrolyte additive composition including a borate-based lithiumcompound represented by Formula 1; and a nitrile-based compoundrepresented by Formula 2, wherein the electrolyte additive compositiondoes not include a phosphate-based compound.

In Formula 1, Y₁ to Y₄ are each independently oxygen (O) or sulfur (S).N≡C—R₁—C≡N   [Formula 2]

In Formula 2, R₁ is an alkylene group having 1 to 5 carbon atoms.

Advantageous Effects

An electrolyte additive composition of the present invention may improvehigh-rate charge and discharge characteristics of a lithium secondarybattery, may suppress an amount of gas generated during high-temperaturestorage, and may improve life characteristics at high temperatureoperation due to a synergistic effect of a novel electrolyte additiveand a nitrile-based compound mixed therewith.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed in more detail. It will be understood that words or terms usedin the specification and claims should be interpreted as having ameaning that is consistent with their meaning in the context of therelevant art and the technical idea of the invention, based on theprinciple that an inventor may properly define the meaning of the wordsor terms to best explain the invention.

Accordingly, since configurations illustrated in examples described inthe specification are merely the most exemplary embodiments of thepresent invention and do not represent the entire technical idea of thepresent invention, it should be understood that there may be variousequivalents and modifications capable of replacing them at the time ofapplication.

Electrolyte Additive Composition

According to the present specification, a novel electrolyte additivecomposition is provided, and the electrolyte additive compositionincludes a borate-based lithium compound represented by Formula 1; and anitrile-based compound represented by Formula 2, wherein the electrolyteadditive composition does not include a phosphate-based compound.

In Formula 1, Y₁ to Y₄ are each independently oxygen (O) or sulfur (S).N≡C—R₁—C≡N   [Formula 2]

In Formula 2, R₁ is an alkylene group having 1 to 5 carbon atoms.

1) Borate-Based Lithium Compound

According to the present specification, a borate-based lithium compoundrepresented by the following Formula 1 is included in the electrolyteadditive composition.

In Formula 1, Y₁ to Y₄ are each independently O or S. Preferably, Y₁ toY₄ may be equally O.

The borate-based lithium compound may be included as an additive in anelectrolyte, wherein the borate-based lithium compound forms uniform andthin films on a positive electrode and a negative electrode, and,particularly, the borate-based lithium compound may improve durabilityof a battery by mainly forming solid electrolyte interface (SEI) on apositive electrode to reduce a reaction between the positive electrodeand other materials and thus forming a uniform and thin film. Also, theborate-based lithium compound may form a robust SEI on a surface of thenegative electrode during the operation of the battery, and high-ratecharge and discharge characteristics of the battery may be improved dueto the interface robustly formed as described above.

As a specific example, the borate-based lithium compound as describedabove may include a compound represented by the following Formula 1a.

The borate-based lithium compound may be appropriately used according toan amount of the electrolyte additive generally added to theelectrolyte, and, for example, may be used in an amount of about 0.01part by weight to about 2 parts by weight, preferably 0.01 part byweight to 0.5 part by weight or 0.1 part by weight to 2 parts by weight,and more preferably 0.5 part by weight to 1 part by weight based on 100parts by weight of a total weight of the electrolyte. In a case in whichthe borate-based lithium compound is used in an amount within the aboverange, a robust SEI may be stably formed on the negative electrode asdescribed above, and the resulting effect may be obtained.

2) Nitrile-Based Compound

According to the present specification, a nitrile-based compound isincluded in the electrolyte additive composition.

The nitrile-based compound is a compound represented by the followingFormula 2.N≡C—R₁—C≡N   [Formula 2]

In Formula 2, R₁ is an alkylene group having 1 to 5 carbon atoms.

The nitrile-based compound is a compound containing two nitrile groups,wherein a linking group linking the two nitrile groups is an alkylenegroup and a carbon number may be in a range of 1 to 5, for example, 2 to4.

The nitrile-based compound, in which the carbon number of the alkylenegroup, as a linking group, is in a range of 2 to 4, includessuccinonitrile, glutaronitrile, or adiponitrile, and at least one ofthese compounds may be included as one component of the electrolyteadditive composition. Among them, succinonitrile or adiponitrile may bepreferably used.

The nitrile-based compound is a compound capable of providing asynergistic effect with the borate-based lithium compound as describedabove on the improvement in performance of the lithium secondarybattery, wherein the nitrile-based compound, as an additive generallyused in a non-aqueous electrolyte of a lithium secondary battery, mayhave an effect such as suppression of the dissolution of positiveelectrode transition metals, and, when it is used with the borate-basedlithium compound, an effect, such as improvement of high-temperaturecharacteristics, may be expected due to the stabilization of thepositive electrode/negative electrode films.

That is, the nitrile-based compound may play a complementary role informing the negative electrode SEI in addition to the effect induced bythe borate-based lithium compound, may play a role in suppressing thedecomposition of a solvent in the electrolyte, and may play a role inimproving mobility of lithium ions.

In a case in which the nitrile-based compound is included in theelectrolyte, the nitrile-based compound may be used in an amount ofabout 0.01 part by weight to about 10 parts by weight, preferably 0.01part by weight to 5 parts by weight or 0.1 part by weight to 10 parts byweight, and more preferably 0.1 part by weight to 5 parts by weightbased on 100 parts by weight of the total weight of the electrolyte.

As described above, the borate-based lithium compound represented byFormula 1 forms uniform and thin films on the positive electrode and thenegative electrode, and particularly, the borate-based lithium compoundimproves durability of the battery by mainly forming the positiveelectrode solid electrolyte interface (SEI) to reduce the positiveelectrode reaction of other materials and thus forming a uniform andthin film.

Upon activation, the borate-based lithium compound may first form theSEI of a negative electrode inorganic component to improve conductivityof lithium cation and form a film having excellent durability, but it isdifficult to obtain the above-described effect by using a singlematerial. Thus, since an electrolyte additive composition is achieved bymixing the borate-based lithium compound represented by Formula 1 withthe nitrile-based compound that may help to form the negative electrodefilm, the SEIs of the positive electrode and the negative electrode arestabilized to improve overall performance, such as high-rate charge anddischarge characteristics, high-temperature storage characteristics, andlife characteristics, of the lithium secondary battery.

Since the film is stably formed as described above, a side reaction, forexample, the decomposition of the solvent in the electrolyte around theelectrode, may be suppressed, and, accordingly, an amount of gasgenerated may be significantly reduced even if the battery is stored ina high-temperature environment. Life performance may also be improveddue to the improvement in storage characteristics.

Furthermore, since the borate-based lithium compound may be stablewithout being decomposed at high temperature, there is no side reaction,for example, decomposition of the surface of the positive electrode oroxidation of the electrolyte. Thus, an increase in irreversible capacityof the battery may be prevented, and accordingly, an effect ofincreasing reversible capacity may be obtained.

3) Others

The electrolyte additive composition according to the presentspecification does not include a phosphate-based compound. Specifically,the phosphate-based compound may be a compound represented by Formula 3below.

In Formula 3, A₁ to A₃ are each independently —Si(R₂)_(n)(R₃)_(3-n), ora propynyl group (—C≡C), wherein R₂ and R₃ are each independently analkyl group having 1 to 4 carbon atoms, and n is an integer of 0 to 3.

The phosphate-based compound, for example, may includetris(trimethylsilyl) phosphate, tris(triethylsilyl) phosphate,tris(tripropylsilyl) phosphate, bis(trimethylsilyl)(triethylsilyl)phosphate, bis(triethylsilyl)(trimethylsilyl) phosphate,bis(tripropylsilyl)(trimethylsilyl) phosphate, andbis(tridimethylsilyl)(tripropylsilyl) phosphate.

The phosphate-based compound as described above may have a significantlyadverse effect on aging characteristics of the electrolyte itself, andthere is a concern that it may interfere with the synergistic effect ofthe borate-based lithium compound and the nitrile-based compound inaddition to the adverse effect on the aging characteristics. Thus, it isdesirable not to include the phosphate-based compound in terms ofimproving high-temperature performance of the battery, and,particularly, it is desirable not to include tributyl phosphate ortris(trimethylsilyl) phosphate.

In the electrolyte additive composition according to the presentspecification, a weight ratio of the borate-based lithium compound tothe nitrile-based compound may be in a range of 1:0.5 to 1:6, preferably1:1 to 1:5, and more preferably 1:2 to 1:5.

In a case in which the ratio satisfies the above range, since a capacityretention at high temperature is improved and the amount of gasgenerated during high-temperature storage is suppressed, an improvementin high-temperature storage characteristics may be expected. That is, anelectrolyte capable of satisfying both battery performance and storagecharacteristics at high temperature may be obtained by adjusting theweight ratio of the borate-based lithium compound to the nitrile-basedcompound, and obtainability may be higher as the ratio is within apreferred range among the above ranges.

Also, the electrolyte additive composition may further include afluorocarbonate-based compound, and the fluorocarbonate-based compound,for example, may include fluoroethylene carbonate or difluoroethylenecarbonate, and may particularly include fluoroethylene carbonate.

In a case in which the fluorocarbonate-based compound is included in theelectrolyte, the fluorocarbonate-based compound may be used in an amountof about 0.01 part by weight to about 10 parts by weight, preferably0.01 part by weight to 5 parts by weight or 0.1 part by weight to 10parts by weight, and more preferably 0.1 part by weight to 5 parts byweight based on 100 parts by weight of the total weight of theelectrolyte. The fluorocarbonate-based compound may be included in aweight ratio of 1:0.5 to 1:6, for example, 1:1 to 1:4 with respect tothe borate-based lithium compound within the above amount range.

In a case in which the fluorocarbonate-based compound, particularly,fluoroethylene carbonate is included within the above range, it isadvantageous in that an improvement in capacity retention at hightemperature, that is, high-temperature performance characteristics maybe additionally obtained.

4) Other Additives

The electrolyte additive composition according to the presentspecification may further include other additives in addition to theborate-based lithium compound represented by Formula 1, thenitrile-based compound represented by Formula 2, and thefluorocarbonate-based compound.

As the additives that may be further included, a carbonate-basedcompound, a borate-based compound, a sulfite-based compound, asultone-based compound, a sulfone-based compound, or afluorobenzene-based compound may be used, and, a mixture of two or moreselected from these compounds may be used.

As the carbonate-based compound, vinylene carbonate or vinyl ethylenecarbonate, for example, may be used, and the compound may be substitutedwith a substituent such as an alkyl group having 1 to 3 carbon atoms.Also, the fluorobenzene-based compound may be a benzene compound, whichis substituted with fluorine instead of hydrogen, such as fluorobenzene,difluorobenzene, and trifluorobenzene.

The borate-based compound may be represented by Formula 4 below.

In Formula 4, A₄ to A₆ are each independently —Si(R₂)_(m)(R₃)_(3-m), ora propynyl group (—C≡C), wherein R₂ and R₃ are each independently analkyl group having 1 to 4 carbon atoms, and m is an integer of 0 to 3.

As the borate-based compound, for example, tris(trimethylsilyl) borate,tris(triethylsilyl) borate, tris(tripropylsilyl) borate,bis(trimethylsilyl)(triethylsilyl) borate,bis(triethylsilyl)(trimethylsilyl) borate,bis(tripropylsilyl)(trimethylsilyl) borate, andbis(tridimethylsilyl)(tripropylsilyl) borate may be used, and thecompound, in which alkyl groups of each silyl group are different fromeach other, may be used.

Also, as the borate-based compound, dipropynyl ethyl borate or diethylpropynyl borate may be used.

Since the borate-based compound promotes ion-pair separation of alithium salt, the borate-based compound may improve mobility of lithiumions, may reduce interfacial resistance of the SEI, and may dissociate amaterial, such as LiF, which may be formed during a battery reaction butis not well separated, and thus, a problem, such as generation ofhydrofluoric acid gas, may be solved.

The sulfite-based compound, the sultone-based compound, and thesulfone-based compound may be represented by Formula 5 below.

In Formula 5, Y₅ and Y₆ are each independently a direct bond, carbon(C), or O, R₅ and R₆ are each independently a substituted orunsubstituted alkyl group having 1 to 10 carbon atoms, a substituted orunsubstituted aryl group having 6 to 20 carbon atoms, or are linkedtogether to form 4- to 7-membered rings, and n is 1 or 2.

In Formula 5, if n is 1, the number of S═O bonds is 1, R₅ and R₆ arelinked together to form a ring, and simultaneously, when Y₅ and Y₆ areO, the compound may be a sulfite-based sulfur-containing compound. If nis 2, the number of S═O bonds is 2, R₅ and R₆ are linked together toform a ring, and simultaneously, when any one of Y₅ and Y₆ is carbon andthe other is oxygen, the compound may be a sultone-basedsulfur-containing compound. Also, if n is 2, the number of S═O bonds is2 and, when R₅ and R₆ do not form a ring, the compound may be asulfone-based compound.

As a specific example, methylene sulfite, ethylene sulfite, trimethylenesulfite, tetramethylene sulfite, or a sulfite having a substituentbonded to these alkylene groups may be used as the sulfite-basedsulfur-containing compound.

Also, as the sulfone-based sulfur-containing compound, dialkyl sulfoneto which an alkyl group having 1 to 5 carbon atoms is bonded, diarylsulfone to which an aryl group having 6 to 12 carbon atoms is bonded, orsulfone having a substituent bonded to the dialkyl or diaryl may beused. And, as the sultone-based sulfur-containing compound, 1,3-propanesultone, 1,3-propene sultone, 1,4-butane sultone, 1,5-pentane sultone,or sultone having a substituent bonded to these alkylene groups may beused.

The sulfite-based, sultone-based, and sulfone-based compounds maygenerally play a role in complementing the formation of the SEI on thesurface of the negative electrode, and the sulfite-based, sultone-based,and sulfone-based compounds may have an effect on high-temperaturestorage performance and high-rate charge and discharge characteristicsby contributing the formation of stable SEI similar to theabove-described borate-based lithium compound.

Electrolyte for Lithium Secondary Battery

According to the present specification, an electrolyte for a lithiumsecondary battery including the above-described electrolyte additivecomposition; a lithium salt; and a non-aqueous organic solvent isprovided.

Since descriptions of the electrolyte additive composition overlap withthose described above, the descriptions thereof will be omitted.

However, with regard to the amount of the electrolyte additivecomposition, the electrolyte additive composition may be included in anamount of 0.1 wt % to 15 wt %, preferably 0.5 wt % to 10 wt %, and morepreferably 1.0 wt % to 7.0 wt % based on the total weight of theelectrolyte.

That is, in order for the nitrile-based compound to complement theborate-based lithium compound and have a synergistic effect, eachcompound may be included in an amount of at least 0.1 wt % or more, and,in a case in which each compound is included in an amount of greater 15wt %, since amounts of the organic solvent and the lithium salt may berelatively reduced, it may degrade basic performance of the batterybeyond the role of the additive. Thus, it is necessary to appropriatelyadjust the amount within a range of 0.1 wt % to 15 wt %, if possible.

In the electrolyte according to the present specification, thenon-aqueous organic solvent may include any kind of organic solventwhich may be used as a non-aqueous electrolyte during the preparation ofa typical lithium secondary battery. In this case, the amount thereofmay be appropriately changed within a normally usable range.

Specifically, the non-aqueous organic solvent may include conventionalorganic solvents, which may be used as a non-aqueous organic solvent ofa lithium secondary battery, such as a cyclic carbonate solvent, alinear carbonate solvent, an ester solvent, or a ketone solvent, and onealone or a mixture of two or more thereof may be used.

The cyclic carbonate solvent may include one selected from the groupconsisting of ethylene carbonate (EC), vinylene carbonate (VC),fluoroethylene carbonate (FEC), propylene carbonate (PC), and butylenecarbonate (BC), or a mixed solution of two or more thereof.

Also, the linear carbonate solvent may include one selected from thegroup consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC),dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methylpropylcarbonate (MPC), and ethylpropyl carbonate (EPC), or a mixed solution oftwo or more thereof.

Furthermore, the ester solvent may include one selected from the groupconsisting of methyl acetate, ethyl acetate, propyl acetate, methylpropionate, ethyl propionate, γ-butyrolactone, γ-valerolactone,γ-caprolactone, δ-valerolactone, and ε-caprolactone, or a mixed solutionof two or more thereof. Also, poly(methyl vinyl ketone) may be used asthe ketone solvent.

In addition, a mixed organic solvent, in which 3 kinds ofcarbonate-based solvents are mixed, may be used as the non-aqueousorganic solvent, and, it is more desirable to use a ternary non-aqueousorganic solvent. Examples of the compound, which may be used in themixing, may be ethylene carbonate, propylene carbonate, butylenecarbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate,ethylmethyl carbonate, vinylene carbonate, fluoroethylene carbonate,methylpropyl carbonate, or ethylpropyl carbonate, and a mixed solvent,in which 3 kinds selected from the above carbonate compounds are mixed,may be used.

Any lithium salt may be used without limitation as the lithium salt,which may be included in the electrolyte, as long as it may providepredetermined lithium ion conductivity and is typically used in anelectrolyte for a lithium secondary battery, and, for example, at leastone selected from the group consisting of F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻,N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻,(CF₃)₅PF⁻, (CF₃)₆P⁻, F₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and (CF₃CF₂SO₂)₂N⁻ may be used as an anion ofthe lithium salt.

Lithium Secondary Battery

According to the present specification, a lithium secondary batteryincluding the above-described electrolyte for a lithium secondarybattery may be provided, and the lithium secondary battery includes apositive electrode including a positive electrode active material, anegative electrode including a negative electrode active material, aseparator disposed between the positive electrode and the negativeelectrode, and the above-described electrolyte.

The lithium secondary battery of the present invention may be preparedaccording to a typical method known in the art. For example, anelectrolyte assembly is formed by sequentially stacking the positiveelectrode, the negative electrode, and the separator disposed betweenthe positive electrode and the negative electrode, and the lithiumsecondary battery may be prepared by injecting an electrolyte in which alithium salt is dissolved.

The positive electrode may be prepared by a typical method known in theart. For example, a binder, a conductive agent, and a dispersant, ifnecessary, as well as a solvent are mixed with a positive electrodeactive material and stirred to prepare a slurry. The positive electrodemay be prepared by coating a slurry on the metal current collector,pressing and then drying.

The positive electrode is prepared by a process of coating the positiveelectrode collector with the positive electrode active material and thendrying the coated positive electrode collector. In this case, alithium-containing transition metal oxide is preferably used as thepositive electrode active material, and, for example, any one selectedfrom the group consisting of Li_(x)CoO₂(0.5<x<1.3),Li_(x)NiO₂(0.5<x<1.3), Li_(x)MnO₂(0.5<x<1.3), Li_(x)Mn₂O₄(0.5<x<1.3),Li_(x)(Ni_(a)Co_(b)Mn_(c))O₂(0.5<x<1.3, 0<a<1, 0<b<1, 0<c<1, a+b+c=1),Li_(x)Ni_(1-y)Co_(y)O₂(0.5<x<1.3, 0<y<1), LixCo_(1-y)Mn_(y)O₂(0.5<x<1.3,0≤y<1), Li_(x)Ni_(1-y)Mn_(y)O₂(0.5<x<1.3, O≤y<1),Li_(x)(Ni_(a)Co_(b)Mn_(c))O₄(0.5<x<1.3, 0<a<2, 0<b<2, 0<c<2, a+b+c=2),Li_(x)Mn_(2-z)Ni_(z)O₄(0.5<x<1.3, 0<z<2),Li_(x)Mn_(2-z)Co_(z)O₄(0.5<x<1.3, 0<z<2), Li_(x)CoPO₄(0.5<x<1.3), andLi_(x)FePO₄(0.5<x<1.3), or a mixture of two or more thereof may be used.Also, an active material having a high content of a specific transitionmetal, such as xLi₂MO₃(1-x)LiMeO₂ (where M is nickel (Ni), cobalt (Co),or manganese (Mn), Me is two or more transition metals selected from thegroup consisting of Ni, Co, Mn, chromium (Cr), iron (Fe), vanadium (V),aluminum (Al), magnesium (Mg), and titanium (Ti), and x satisfies0<x<1), may be used.

The lithium-containing transition metal oxide may be coated with ametal, such as aluminum (Al), or a metal oxide. Also, in addition to thelithium-containing transition metal oxide, a sulfide, a selenide, or ahalide may be used.

The positive electrode collector is generally formed to a thickness of 3μm to 500 μm. The positive electrode collector is not particularlylimited so long as it has conductivity without causing adverse chemicalchanges in the battery, and any metal may be used as long as it, as ametal with high conductivity as well as a metal to which the slurry ofthe positive electrode active material may be easily adhered, is notreactive in a voltage range of the battery. Non-limiting examples of thepositive electrode collector may be aluminum, nickel, or a foil preparedby combination thereof.

The solvent used for forming the positive electrode may include anorganic solvent, such as N-methylpyrrolidone (NMP), dimethylformamide(DMF), acetone, and dimethylacetamide, or water, and these solvents maybe used alone or in a mixture of two or more thereof.

An amount of the solvent used may be sufficient if the solvent maydissolve and disperse the electrode active material, the binder, and theconductive agent in consideration of a coating thickness of the slurryand manufacturing yield.

The conductive agent may be used without limitation as long as it may begenerally used in the art, and, for example, artificial graphite,natural graphite, carbon black, acetylene black, Ketjen black, Denkablack, thermal black, channel black, carbon fibers, metal fibers,aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium,vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten,silver, gold, lanthanum, ruthenium, platinum, iridium, titanium oxide,polyaniline, polythiophene, polyacetylene, polypyrrole, or a mixturethereof may be used.

The binder may be used without limitation as long as it is generallyused in the art, and, for example, polyvinylidene fluoride (PVDF), apolyvinylidene fluoride-hexafluoropropylene copolymer (PVDF/HFP),poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide,polyvinylpyrrolidone, polyvinylpyridine, alkylated polyethylene oxide,polyvinyl ether, poly(methyl methacrylate), poly(ethyl acrylate),polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, astyrene-butadiene rubber, an acrylonitrile-butadiene rubber, a fluororubber, an ethylene-propylene-diene monomer (EPDM), a sulfonatedethylene-propylene-diene monomer, carboxymethylcellulose (CMC),regenerated cellulose, starch, hydroxypropylcellulose,tetrafluoroethylene, or a mixture thereof may be used.

In the positive electrode, a filler may be further added to the mixture,if necessary. The filler, as a component that suppresses the expansionof the positive electrode, is selectively used, wherein the filler isnot particularly limited as long as it is fibrous material while notcausing chemical changes in the battery, and, for example, anolefin-based polymer such as polyethylene and polypropylene; and afibrous material, such as glass fibers and carbon fibers, are used.

The negative electrode may be prepared by a typical method known in theart. For example, a binder, a conductive agent, and a dispersant, ifnecessary, as well as a solvent are mixed with a negative electrodeactive material and stirred to prepare a slurry, a metal currentcollector is then coated with the slurry and pressed, and the negativeelectrode may then be prepared by drying the coated metal currentcollector.

As the negative electrode active material, amorphous carbon orcrystalline carbon may be included, and, specifically, carbon such asnon-graphitizable carbon and graphite-based carbon; a complex metaloxide such as Li_(x)Fe₂O₃ (0≤x≤1), Li_(x)WO₂ (0≤x≤1),Sn_(x)Me_(1-x)Me′_(y)O_(z) (Me: manganese (Mn), iron (Fe), lead (Pb), orgermanium (Ge); Me′: aluminum (Al), boron (B), phosphorus (P), silicon(Si), Groups I, II and III elements of the periodic table, or halogen;0<x≤1; 1≤y≤3; 1≤z≤8); a lithium metal; a lithium alloy; a silicon-basedalloy; a tin-based alloy; a metal oxide such as SnO, SnO₂, PbO, PbO₂,Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄, and Bi₂O₅; aconductive polymer such as polyacetylene; or a Li—Co—Ni-based materialor a Li—Ti—O-based material may be used.

Those used in the positive electrode may be equally used as the binderand the conductive agent included in the negative electrode.

The negative electrode collector is generally formed to a thickness of 3μm to 500 μm. The negative electrode collector is not particularlylimited so long as it has conductivity without causing adverse chemicalchanges in the battery, and, for example, copper, stainless steel,aluminum, nickel, titanium, fired carbon, copper or stainless steel thatis surface-treated with one of carbon, nickel, titanium, silver, or thelike, an aluminum-cadmium alloy, or the like may be used. Also, similarto the positive electrode collector, the negative electrode collectormay have fine surface roughness to improve bonding strength with thenegative electrode active material, and the negative electrode collectormay be used in various shapes such as a film, a sheet, a foil, a net, aporous body, a foam body, a non-woven fabric body, and the like.

Both of a polyolefin-based polymer typically used in the art and acomposite separator having an organic-inorganic composite layer formedon an olefin-based substrate may be used as the separator disposingbetween the positive electrode and the negative electrode and insulatingthese electrodes, but the separator is not particularly limited thereto.

The positive electrode, negative electrode, and separator, which havethe above-described structure, are accommodated in a pouch case, and apouch type battery may then be prepared by injecting the non-aqueouselectrolyte, but the present invention is not limited thereto. A shapeof the lithium secondary battery according to the present specificationis not particularly limited, but a cylindrical type using a can or aprismatic type may be used, and a coin type may be used.

Application Products

A battery module according to another embodiment of the presentinvention includes the above-described lithium secondary battery as aunit cell, and a battery pack according to another embodiment of thepresent invention includes the battery module.

The lithium secondary battery according to the present invention may notonly be used in a battery module that is used as a power source of asmall device, but may also be used as a unit cell in a medium and largesized battery pack including a plurality of batteries. However, sinceoxidation stability is excellent, the lithium secondary batteryaccording to the present invention may be preferably used as ahigh-voltage power source of a small device, and may be a more suitableelectrolyte system rather than being used as a power source of a mediumand large sized device.

EXAMPLES

Hereinafter, the present invention will be described in more detailaccording to examples. However, the invention may be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein. Rather, these example embodiments areprovided so that this description will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art.

Examples 1 to 6

The borate-based lithium compound represented by the following Formulala and succinonitrile were mixed such that the borate-based lithiumcompound and the succinonitrile were respectively included in amounts of1.0 wt % based on a total electrolyte, and an electrolyte additivecomposition, in which the borate-based lithium compound and thesuccinonitrile were mixed in a weight ratio of 1:1, was prepared inExample 1. The borate-based lithium compound and succinonitrile weremixed in weight ratios as listed in Table 1 to prepare electrolyteadditive compositions of Examples 2 to 6.

Examples 7 and 8

Electrolyte additive compositions of Examples 7 and 8 were respectivelyprepared by further mixing fluoroethylene carbonate in Example 4 inweight ratios as listed in Table 1.

Comparative Examples 1 to 7

Electrolyte additive compositions of Comparative Examples 1 to 7 wereprepared by adjusting types and amounts of additives as listed in Table1 below.

TABLE 1 Additive amount¹⁾ Additive Formula weight Total 1a SN²⁾ FEC³⁾TBP⁴⁾ TMSP⁵⁾ ratio⁶⁾ amount¹⁾ Example 1 1.0 0.5 1:0.5 1.5 Example 2 1.01.0 1:1 2 Example 3 1.0 2.0 1:2 3 Example 4 1.0 3.0 1:3 4 Example 5 1.05.0 1:5 6 Example 6 1.0 6.0 1:6 7 Example 7 1.0 3.0 1.0 1:3:1 4 Example8 1.0 3.0 2.0 1:3:2 6 Comparative 1.0 0.5 1:0.5 1.5 Example 1Comparative 1.0 0.5 1:0.5 1.5 Example 2 Comparative 1.0 3.0 0.5 1:3:0.54.5 Example 3 Comparative 1.0 3.0 0.5 1:3:0.5 4.5 Example 4 Comparative3.0 — 3.0 Example 5 Comparative 1.0 2.0 — 3.0 Example 6 Comparative 1.00.5 — 1.5 Example 7 ¹⁾weight % based on a total weight of theelectrolyte ²⁾SN: succinonitrile ³⁾FEC: fluoroethylene carbonate ⁴⁾TBP:tributyl phosphate ⁵⁾TMSP: tris(trimethylsilyl) phosphate ⁶⁾weightratios of additives based on the borate-based lithium compound ofFormula 1a

Experimental Example Performance Evaluation of Lithium Secondary Battery

(1) Preparation of Electrolyte

1 mole/l of LiPF₆ was added to a non-aqueous organic solvent having acomposition, in which propylene carbonate (PC):ethyl propionate(EP):dimethyl carbonate (DMC)=2:4:4 (weight ratio), based on a totalamount of a non-aqueous electrolyte solution, and the electrolyteadditive compositions of Examples 1 to 8 and Comparative Examples of 1to 7 were added in amounts as listed in Table 1 to prepare electrolytes.

(2) Preparation of Lithium Secondary Battery

89 wt % of a mixture of Li(Ni_(0.33)Co_(0.33)Mn_(0.33))O₂ as a positiveelectrode active material, 8 wt % of carbon black as a conductive agent,and 3 wt % of polyvinylidene fluoride (PVDF), as a binder, were added toN-methyl-2-pyrrolidone (NMP), as a solvent, to prepare a positiveelectrode mixture slurry. An about 20 μm thick aluminum (Al) thin film,as a positive electrode collector, was coated with the positiveelectrode mixture slurry, dried, and then roll-pressed to prepare apositive electrode.

Also, 97 wt % of carbon powder as a negative electrode active material,2 wt % of PVDF as a binder, and 1 wt % of carbon black, as a conductiveagent, were added to NMP, as a solvent, to prepare a negative electrodemixture slurry. A 10 μm thick copper (Cu) thin film, as a negativeelectrode collector, was coated with the negative electrode mixtureslurry, dried, and then roll-pressed to prepare a negative electrode.

After an electrode assembly was prepared by a typical method by stackinga polyolefin separator with the positive electrode and negativeelectrode thus prepared, and the electrolyte prepared in ‘(1)’ wasinjected to complete the preparation of a lithium secondary battery.

Performance evaluation on the following items was performed on thelithium secondary batteries in which the electrolytes of Examples 1 to 8and Comparative Examples of 1 to 7 were included as described above.

(3) Evaluation Items

1) High-Temperature Life Characteristics Evaluation

The lithium secondary batteries, in which the electrolyte additivecompositions of Examples 1 to 8 and Comparative Examples 1 to 7 wereused, were charged at 1.0 C/4.45 V to 4.45 V/112 mA under a constantcurrent/constant voltage (CC/CV) condition at 45° C. and discharged at1.0 C to a voltage of 3.0 V. This charge and discharge cycle wasrepeated 400 times and a capacity retention was calculated usingEquation 1 below.Capacity retention (%)=(discharge capacity after 400 cycles/initialdischarge capacity)×100   [Equation 1]

2) High-Temperature Storage Characteristics Evaluation

(a) Capacity Retention (%)

The lithium secondary batteries, in which the electrolyte additivecompositions of Examples 1 to 8 and Comparative Examples 1 to 7 wereused, were charged at 0.2 C/4.45 V to 4.45 V/112 mA under a constantcurrent/constant voltage (CC/CV) condition at room temperature anddischarged at 0.2 C to a voltage of 3.0 V to measure initial dischargecapacity.

Thereafter, the secondary batteries were charged at 0.2 C/4.45 V to 4.45V/112 mA under a constant current/constant voltage (CC/CV) condition atroom temperature and then stored at 60° C. for 21 days. After thestorage, the secondary batteries were charged at 0.2 C/4.45 V to 4.45V/112 mA under a constant current/constant voltage (CC/CV) condition atroom temperature and discharged at 0.2 C to a voltage of 3.0 V tomeasure discharge capacity after storage. a capacity retention wascalculated using Equation 2 below.Capacity retention (%)=discharge capacity after 21 days storage(mAh)/initial discharge capacity (mAh)   [Equation 2]

(b) Thickness Increase Rate

After the initial charge and discharge in experiment (a), each batterywas set to a state of charge (SOC) of 50% to measure a thickness, andthe thickness was defined as an initial thickness. A battery thickness,which was measured at 60° C. after high-temperature storage at a SOC of100%, was defined as a final thickness, and a thickness increase rate(%) of the battery was calculated using the following Equation 3.Thickness increase rate (%)=(final thickness initial thickness/initialthickness)×100(%)   [Equation 3]

(4) Evaluation Results

Performances of the lithium secondary batteries, in which theelectrolyte additive compositions of Examples 1 to 8 and ComparativeExamples 1 to 7 were used, were evaluated according to the aboveevaluation items, and the results thereof are presented in Table 2below.

TABLE 2 Capacity Capacity Thickness retention retention increase (%) (%)rate (%) 45° C., 60° C., 60° C., 400 cycle 21 days 21 days Example 179.4 76.3 15.2 Example 2 82.5 88.5 12.1 Example 3 84.7 90.7 11.2 Example4 85.2 91.4 10.7 Example 5 83.4 85.9 10.3 Example 6 86.8 87.6 11.8Example 7 88.0 86.4 12.6 Example 8 76.2 80.5 10.4 Comparative Example 170.1 63.2 25.8 Comparative Example 2 69.6 — Vent¹⁾ Comparative Example 374.8 68.9 19.9 Comparative Example 4 75.8 57.3 30.7 Comparative Example5 62.5 75.1 10.2 Comparative Example 6 79.1 — Vent¹⁾ Comparative Example7 72.5 59.8 33.1 ¹⁾vent: a state in which evaluation was not possiblebecause an increase in thickness of the battery was severe due to thegeneration of excessive amount of gas

Referring to Table 2, it may be confirmed that Examples 1 to 8, in whichthe borate-based lithium compound and the nitrile-based compound(succinonitrile) were mixed in a ratio of 1:0.5 to 1:6, were evaluatedas excellent in terms of both high-temperature life characteristics andhigh-temperature storage characteristics in comparison to ComparativeExamples 1 to 7 in which the nitrile-based compound was not used or thephosphate-based compounds were further used.

Specifically, with respect to Comparative Examples 1 and 2 in which thenitrile-based compound was not used, but the phosphate-based compoundswere respectively used with the borate-based lithium compound of Formula1a, it may be confirmed that both life characteristics and storagecharacteristics at high temperature were quite poor, and, particularly,with respect to Comparative Example 2, it may be confirmed that thebattery was malfunctioned because the amount of gas generated wasexcessive.

Also, capacity retentions of Comparative Examples 3 and 4, in which thenitrile-based compound was used, but the phosphate-based compounds wererespectively further used, were relatively better than those ofComparative Examples 1 and 2, but these capacity retentions were alsosignificantly lower than those of Examples, and, particularly, sincecapacity retention and thickness increase rates were poor, it may beconfirmed that high-temperature storage characteristics were poor. Fromthese results, it was confirmed that the phosphate-based compounds werenot suitable as an electrolyte additive.

In addition, with respect to Comparative Example 5, in which thecompound of Formula la was only used, and Comparative Examples 6 and 7in which the compound of Formula 1a was not used, since cycleperformances at high temperature were poor or amounts of gas generatedduring high-temperature storage were excessive, it may be confirmed thatthe batteries were vented or increases in thickness were severe.

Furthermore, with respect to Examples 1 to 8 according to the presentspecification, their effects were obtained by using the borate-basedlithium compound of Formula 1a and the nitrile-based compound in a ratioof 1:0.5 to 1:6 based on the above data, but it may be confirmed that itwas more desirable when the borate-based lithium compound of Formula 1aand the nitrile-based compound were used in a ratio of 1:1 to 1:5, andit was confirmed that, in a case in which the fluoroethylene carbonatewas further used, the high-temperature life characteristics and thehigh-temperature storage characteristics may be further improved.

The invention claimed is:
 1. A lithium secondary battery comprising apositive electrode, a negative electrode, a separator disposed betweenthe positive electrode and the negative electrode, and an electrolyte,wherein the electrolyte comprises a lithium salt, a non-aqueous organicsolvent, and an electrolyte additive composition comprising: aborate-based lithium compound represented by Formula 1a:

and a nitrile-based compound, wherein a weight ratio of the borate-basedlithium compound to the nitrile-based compound is in a range of 1:2 to1:3, wherein the nitrile-based compound is succinonitrile, and whereinthe electrolyte additive composition does not comprise a phosphate-basedcompound.
 2. The lithium secondary battery of claim 1, wherein theelectrolyte additive composition further comprises fluoroethylenecarbonate.
 3. The lithium secondary battery of claim 1, wherein theelectrolyte additive composition is included in an amount of 0.1 wt % to15 wt % based on a total weight of the electrolyte.
 4. The lithiumsecondary battery of claim 1, wherein the borate-based lithium compoundis included in an amount of about 0.01 part by weight to about 2 partsby weight based on 100 parts by weight of a total weight of theelectrolyte.
 5. The lithium secondary battery of claim 1, wherein thenitrile-based compound is included in an amount of about 0.01 part byweight to about 10 parts by weight based on 100 parts by weight of atotal weight of the electrolyte.
 6. The lithium secondary battery ofclaim 2, wherein the fluoroethylene carbonate is included in an amountof about 0.01 part by weight to about 10 parts by weight based on 100parts by weight of the total weight of the electrolyte.
 7. The lithiumsecondary battery of claim 1, wherein the electrolyte additivecomposition further comprises at least one additive selected from thegroup consisting of a carbonate-based compound, a borate-based compound,a sulfite-based compound, a sultone-based compound, a sulfone-basedcompound, and a fluorobenzene-based compound.
 8. The lithium secondarybattery of claim 1, wherein the non-aqueous organic solvent comprises atleast one selected from the group consisting of a cyclic carbonatesolvent, a linear carbonate solvent, an ester solvent, and a ketonesolvent.
 9. The lithium secondary battery of claim 1, wherein thelithium salt comprises at least one selected from the group consistingof F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻,(CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, F₃SO₃ ⁻, CF₃CF₂SO₃ ⁻,(CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SF₅)₃C⁻,(CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and(CF₃CF₂SO₂)₂N⁻ as an anion.